Design procedure – Rainbow Electronics MAX8775 User Manual
Page 23
Thermal-Fault Protection
The MAX8775 features a thermal-fault protection circuit.
When the junction temperature rises above +160°C, a
thermal sensor sets the fault latches, pulls PGOOD_
low, and shuts down both SMPS controllers using the
soft-shutdown sequence (see the
Soft-Start and Soft-
Shutdown
section). Cycle V
CC
below 1V or toggle ON1
and ON2 to clear the fault latches and restart the con-
trollers after the junction temperature cools by 15°C.
Design Procedure
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency
and inductor operating point (ripple-current ratio). The
primary design trade-off lies in choosing a good switch-
ing frequency and inductor operating point, and the fol-
lowing four factors dictate the rest of the design:
•
Input Voltage Range. The maximum value
(V
IN(MAX)
) must accommodate the worst-case, high
AC-adapter voltage. The minimum value (V
IN(MIN)
)
must account for the lowest battery voltage after
drops due to connectors, fuses, and battery selector
switches. If there is a choice at all, lower input volt-
ages result in better efficiency.
•
Maximum Load Current. There are two values to
consider. The peak load current (I
LOAD(MAX)
)
determines the instantaneous component stresses
and filtering requirements and thus drives output
capacitor selection, inductor saturation rating, and
the design of the current-limit circuit. The continu-
ous load current (I
LOAD
) determines the thermal
stresses and thus drives the selection of input
capacitors, MOSFETs, and other critical heat-con-
tributing components.
•
Switching Frequency. This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage, due to MOSFET switching losses that
are proportional to frequency and V
IN
2
. The opti-
mum frequency is also a moving target, due to
rapid improvements in MOSFET technology that are
making higher frequencies more practical.
•
Inductor Operating Point. This choice provides
trade-offs between size and efficiency and between
transient response and output ripple. Low inductor
values provide better transient response and small-
er physical size, but also result in lower efficiency
and higher output ripple due to increased ripple
currents. The minimum practical inductor value is
one that causes the circuit to operate at the edge of
critical conduction (where the inductor current just
touches zero with every cycle at maximum load).
Inductor values lower than this grant no further size-
reduction benefit. The optimum operating point is
usually found between 20% and 30% ripple current.
When pulse skipping (
SKIP_ low and light loads),
the inductor value also determines the load-current
value at which PFM/PWM switchover occurs.
Inductor Selection
The per-phase switching frequency and inductor oper-
ating point determine the inductor value as follows:
For example: I
LOAD(MAX)
= 15A, V
IN
= 12V, V
OUT
=
1.5V, f
OSC
= 300kHz, 30% ripple current or LIR = 0.3:
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. For the
selected inductance value, the actual peak-to-peak
inductor ripple current (ΔI
INDUCTOR
) is defined by:
Ferrite cores are often the best choice, although pow-
dered iron is inexpensive and can work well at 200kHz.
The core must be large enough not to saturate at the
peak inductor current (I
PEAK
):
Transient Response
The inductor ripple current also impacts transient-
response performance, especially at low V
IN
- V
OUT
dif-
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The total output voltage sag is the sum of the voltage
sag while the inductor is ramping up, and the voltage
sag before the next pulse can occur:
where D
MAX
is the maximum duty factor (see the
Electrical Characteristics
), T is the switching period (1 /
f
OSC
), and ΔT equals V
OUT
/ V
IN
x T when in PWM
mode, or L x 0.2 x I
MAX
/ (V
IN
- V
OUT
) when in skip
V
L
I
C
V
D
V
I
T
T
C
SAG
LOAD MAX
OUT
IN
MAX
OUT
LOAD MAX
OUT
=
(
)
×
−
(
)
+
−
(
)
Δ
Δ
Δ
(
)
(
)
2
2
I
I
I
PEAK
LOAD MAX
INDUCTOR
=
+
(
)
Δ
2
ΔI
V
V
V
V f
L
INDUCTOR
OUT
IN
OUT
IN OSC
=
−
(
)
L
V
V
V
V
kHz
A
H
=
Ч
−
(
)
Ч
Ч
Ч
=
1 8
12
1 8
12
300
15
0 3
0 97
.
.
.
.
μ
L
V
V
V
V f
I
LIR
OUT
IN
OUT
IN OSC LOAD MAX
=
−
(
)
(
)
MAX8775
Dual and Combinable Graphics Core
Controller for Notebook Computers
______________________________________________________________________________________
23